The Lucy UI is composed of a number of panels.
For example the map is contained in a panel, the layer control in another panel, the object properties in yet
another panel, ... .
These panels are called ILcyApplicationPane instances in the Lucy API.
Application panes make abstraction of the actual form of these panels: they can be tabbed panes, internal frames, panels inside a bigger whole or anything else you can come up with. In the out-of-the-box setup, the application panes are the dockable frames inside the Lucy frame.
This abstraction is useful because it allows you to change the Lucy frontend, which defines the form of the application panes, while add-ons can just work with the application panes without knowing where and how they are displayed.